The OOXML specification, for spreadsheet applications like Excell 2007, ignores the fact that the week-end is not Saturday and Sunday in the Arab world and other possible countries.

An example of a concern is the spreadsheet function NETWORKDAYS()25. This function is defined by OOXML to return the number of working days between two dates, exclusive of any weekends in that interval. For some cultures, the weekend is Saturday and Sunday. For others, the days of rest are either Thursday/Friday or Friday/Saturday. OOXML does not define "weekend" and does not provid a way for the user to define it either. As implemented in Excel the function assumes the weekend is always Saturday/Sunday. This spreadsheet function is defined in a way which renders an incorrect answer for potentially billions of people across the globe. OOXML lacks cultural adaptability. Compare this to the same function in OpenDocument Format, where the user may pass in an additional parameter to override the default definition of a weekend.
24 Part 4, Section 4.7.1
25 Part 4, Section 3.17.7.224"

Concerned countries that are members of ISO are the following (P-members are in bold):

Azerbaijan

Iraq

Iran (not part of the Arabic world, have to find out when are the days off of the week)